During membrane filtration processes, a feed liquid containing undesirable contaminants is forced through the pores of a membrane. The contaminants contained in the feed liquid do not pass through the pores of the membrane and accumulate on the outside of the membranes in the membrane module. Those particles cause membrane fouling and can clog membrane modules, making them less efficient. Liquid backwash, air scouring and chemical cleaning are common means to remove foulants from the module. However, the membranes in the modules still often suffer from packing by fibrous materials or damage by sharp objects contained within the feed liquid. Fibrous materials or trash may contact the membranes and may become caught around bundles or groups of membranes. In particular, when membranes are used for the pretreatment seawater, sharp shells from sea water often contact the membranes and cause damage to them.
Different methods have been proposed to protect the membrane modules from damage by materials and objects contained within a feed liquid. Pre-screening feed liquid is a common method employed to protect membrane modules from damage by materials in the feed liquid having a size greater than the pores of the membrane fibers. For example, in membrane bioreactors, a pre-treatment screen with perforated mesh is sometimes inserted between the biological tank and the membrane tank to protect the membranes and modules. In this arrangement however, a large pre-treatment screen has to be used and special aerators need to be installed to clean the pre-treatment screen. The large size of the screen and special aerators required to clean the screen present obstacles to efficiently operating such a bioreactor system.
Another method of protecting membranes includes holding a bundle of capillaries together with an open sleeve or casing. Coarse materials that may be contained in a feed liquid, such as hair, are filtered by the sleeve so that these materials can not intrude into the inner section of the membrane bundle. However, the method neglects to remove particles that are smaller than the sleeve opening. Small particles can pass through the open sleeve, move towards the membrane surface and inner fiber bundles, and agglomerate around and inside the membrane fiber bundles. It is difficult for these materials to be removed out of the open sleeve. Gradually these materials accumulate inside the membrane bundle and on the inner surface of the sleeve and the bundles eventually become packed with these materials. The accumulation of these materials can also cause membrane damage. The inability to effectively remove these materials when using an open sleeve configuration presents an obstacle to efficient filtration.